The present disclosure relates generally to a method and apparatus for cooling an extrudate or extruded articles. Specifically, the present disclosure relates to circulating a cryogen through a cooling chamber including sizing and/or calibration tools, through a hollow in the article itself or a combination of the aforementioned to cool an extrudate. Additionally, many other applications of the invention will become apparent to those skilled in the art upon a review of the following specification and drawings.
Extruded materials, for example products comprising plastics, rubbers, wood composites, etc., are formed by mixing raw materials under high temperature and pressure and passing the mixture through a die to form the final shape. The extruded material, or extrudate, is subject to deformation after leaving the extruder because of the plastic material properties at high temperature. The extrudate must be cooled to provide rigidity for further operation. If the extrudate is not quickly and effectively cooled, the extrudate may deform, leading to rejection of the extruded products.
Certain continuously extruded materials, e.g., rubber products, plastic products, metal products, wood composites, must be cooled after passing through the extrusion operation in order to prevent deformation. In conventional extrusion operations, the extruded materials, be it hose, pipe, rod, bar or any other shape may deform from its own weight if the temperature was not decreased rapidly after leaving the extruder. Cooling the product rapidly creates, at least, a minimum amount of rigidity in the extrudate such that the manufacturer can cut, stack or otherwise handle the extrudate without unwanted deformation. If the product is not cooled effectively and quickly, the resultant deformation can lead to excessive rates of rejection of the manufactured or extruded product. Further, the rate at which the extrudate is cooled directly affects the rate at which product may be produced. In other words, the faster an extrudate is cooled, the faster the end product can be produced.
Historically, cooling water systems have been utilized as the primary medium for cooling articles, including extrusions. For example, conventional extrusion chilling systems employ a “cooling” chamber downstream from the extruder. The extrusion is fed through the cooling chamber, wherein the extrusion can be sprayed with water, or partially/fully submerged in water in order to chill the extrusion. Various other components may also be included in such systems, such as a vacuum sizing chamber intermediate to the extruder and the cooling chamber. The vacuum sizing chamber can be used for both solid and hollow extrusions and employs an external vacuum pump to create a vacuum to assist the extrusion in maintaining its shape while it cools. Water can also be used in the vacuum chamber to cool the extrusion while the vacuum supports the shape.
However, cooling water systems have several drawbacks. Many products are adversely affected if contacted with water. Thus extra care must be taken to avoid such occurrences. Extrusion speeds are limited because the cooling water generally has a well defined heat transfer capability and thus can only cool the fresh extrudate in accordance therewith. In practice, an optimum cooling temperature of approximately 50° F. is achievable from a cost-effective standpoint, which limits the manufacturer's ability to cool extrusions quickly. Additionally, cooling water systems require excessive floor space and also require treatments or special additive packages to prepare and maintain proper water chemistry, as well as to prevent scaling and bacterial growth, which add significantly to the cost thereof.
Coolant mediums other than water which have been used in cooling processes can be referred to collectively as refrigerants, including cryogens. Cryogens include liquid nitrogen, liquid carbon dioxide, liquid air and other refrigerants having normal boiling points substantially below minus 50° F. (−46° C.). Prior art methods of cooling articles using cryogens disclose the benefits of fully vaporizing a cryogen into a gaseous refrigerant prior to contact with the articles to be cooled. Cryogens due to their extremely low boiling point, naturally and virtually instantaneously expand into gaseous form when dispersed into the air. This results in a radical consumption of heat. The ambient temperature can be reduced to hundreds of degrees below zero (Fahrenheit) in a relatively short time, and much quicker than may be realized with a conventional cooling water system. The extreme difference in vaporized cryogen and the extruded product allows the manufacturer to quickly cool an extrudate.
However, prior methods of cryogenic cooling fail to realize the advantages, both in increased efficiency and in improved system control, that can be achieved by utilizing forced gas convection in combination with cryogenic refrigerants, such as nitrogen and air. Some disadvantages of prior art cryogenic cooling systems include lower efficiency and limited options for controlling the cooling process. Such systems generally rely exclusively on the cooling effect of the refrigerant, to lower the ambient temperature and chill the article. Although prior art methods utilize forced convection to ensure complete vaporization of the cryogen, no methods use forced gas convection to control the rate of cooling of the article by controlling the wind chill temperature. Consequently, the only control variable in the prior art methods to adjust (lower) the temperature is the introduction of a liquid cryogen into the system. In contrast, utilization of forced gas convection adds a wide range of variable control to adjust the effective temperature, up or down, by controlling the velocity at which the refrigerant is circulated over/around the article to be cooled. Such a forced gas convection system is disclosed by Thomas et al. in U.S. Pat. No. 6,363,730, U.S. Pat. No. 6,389,828, and U.S. Patent Application Publication No. 2004/0216470 (now abandoned) incorporated herein in their entirety by reference thereto.
The basis of forced gas convection is the principle that increasing velocity of a refrigerant over a heated surface, such as by blowing, greatly enhances the transfer of heat from that surface. In the context of cold temperatures, this principle is probably better known indirectly from the commonly used phrase “wind chill” temperature. In that context, wind chill temperature is the apparent temperature to human flesh, taking into account the ambient temperature and the prevailing velocity of the wind. The stronger (higher velocity) the wind, the lower the temperature “feels,” compared to if there were no wind present. Forced gas convection cooling systems, as disclosed herein, take advantage of this “wind chill” affect in their ability to remove heat from an object faster with a constant temperature of a gas.
In other words, if a 400° F. object is placed in a constant 75° F. atmosphere without velocity of the surrounding atmosphere, the transfer of energy from the object to the surrounding atmosphere by convection is much slower than if the atmosphere has a velocity over/around the object. An increase in velocity will increase the rate of energy transfer, even though the temperature of the atmosphere is constant. The rate of cooling can be increased or decreased by manipulating the velocity of the cooling medium as the temperature of the medium remains constant. This principle is advantageously utilized to significantly enhance the cooling efficiency of the system by creating, and controlling, “wind chill” temperature during the cooling process. As a result, the efficiency of the process is increased while simultaneously reducing the size, which is typically the length, of the cooling system.
However, the previous method disclosed by Thomas utilizes only a measurement of the ambient temperature within the cooling chamber to adjust the velocity and discharge of cryogen. An extrudate leaving a cooling chamber does not necessarily need to be cooled to an even temperature throughout, but may rely on “equilibrium cooling.” This principle is advantageously utilized according to the invention to significantly enhance the cooling efficiency of the system by creating and controlling the “wind chill” temperature during the cooling process in relation to a measurement of the temperature of the product after leaving the cooling chamber. The basis for “equilibrium cooling” is that a mass having two different temperature zones, or a temperature gradient, will exchange energy between the two zones until an “equilibrium” temperature is reached. Thus, a manufacturer can reduce cooling time and cooling system length by super-cooling at least the surface of the extrudate mass to form a “skin” having sufficient rigidity such that the extrudate may be handled as needed and then allowing the “equilibrium cooling” effect to take place after the extrudate has left the cooling system.
Another type of prior art cooling system utilizes a device called a “calibrator,” and typically multiple such calibrators, to cool extrusions. A calibrator is a tool which generally has a central opening through which the extrusion is fed, the central opening having a surface which is generally in contact with the surface of the extrusion as it is fed through. As a result of contact with the surface of the extrusion, the calibrator acts as a heat sink and the heat is conducted to the calibrator and away from the extrusion thus cooling the extrusion. Since cooling of the extrudate tends to make the material contract or change shape, a vacuum generated by external vacuum pumps is generally drawn through grooves in the calibrator inner surface making contact with the extrudate. This vacuum assists in maintaining the shape of the extrudate.
To enhance the heat transfer from the extrusion, internal passages or circuits are provided in the calibrator through which a coolant is circulated. Typically, the coolant is water, but liquid nitrogen is also known to have been used to some degree. However, circulating liquid nitrogen through the cooling circuits has met with some difficulties regarding contact of the liquid nitrogen with the calibrators. Additionally, cooling water systems include the inherent problems associated therewith as discussed above. The aforementioned U.S. Pat. No. 6,389,828 to Thomas discloses that it is preferable to first vaporize a liquid cryogen, such as liquid nitrogen, and then to circulate the super-cold vapor/refrigerant through the cooling circuits instead of the liquid cryogen, which thus requires a system for vaporizing the liquid cryogen prior to circulation through the cooling circuits of the calibrator.
Although such a method is an improvement over the prior art, the system may still require the use of external vacuum pumps as previously stated. The present invention provides for a calibration tooling chamber utilizing forced-gas convection of a cryogenic refrigerant in combination with a calibrator tooling or sizing template having a plurality of fins in an outer surface thereof to allow the extrudate to be cooled at an effective rate. This eliminates the need for internal passages, and thus the additional manufacturing costs associated with the required set-up/connection/break-down of the equipment between different product runs. Further, the present disclosure, by use of a forced gas convection cooling chamber, provides a means of generating an internally induced vacuum to assist the extrudate without the requirement of a separate external pump. External vacuum pumps are expensive, require continued maintenance and repair, are noisy and they must be replaced often.
Many extruded articles include at least one hollow, such as pipe, hose, etc., or may contain several hollow portions. Some cooling systems provide the manufacturer with only the ability to cool an extrudate from an outer surface thereof by contact with a cooler medium (liquid, gas or solid depending on the system). Depending on the product geometry, however, a significant amount of an extrudate's mass may be positioned inward of the outer surface and between several hollow portions. Thus, it is difficult to quickly and effectively cool such an extrudate quickly because the cooling medium does not make contact with those portions.
Another problem encountered with cryogenic cooling systems, especially those that are separated-air gasses such as nitrogen, is the loss of cryogenic coolant. Cryogenic cooling systems are usually pressurized and cryogen is lost through system leaks or through cryogen being expelled into the hollow of an extruded article and exhausted to atmosphere. Such loss of cryogen must be replaced within the system and can be expensive.
Accordingly, there is a need for a method and apparatus for cooling articles which can provide improved efficiency, reduce the size of the cooling system.